1. Field of the Invention
This invention relates to a novel composite membrane of the so-called functional membrane which is very stable to heat and chemicals and has high and selective molecular sieve activity and/or catalytic activity, and to processes for the production of the composite membrane.
2. Description of the Prior Art
Naturally occurring zeolites were already known before 1930. Synthetic zeolites having molecular sieve activity were later studied, notably by a systematic and comprehensive work of R. M. Barrer and his coworkers which began in the middle of 1940s. The outcome of this work is seen, for example, in R. M. Barrer: "Molecular Sieves", page 39, published by Society of the Chemical Industry (1968). The research workers of the Linde Division of Union Carbide Corporation who were motivated by the research results of Barrer et al. succeeded in the commercial production of a molecular sieve zeolite in a single crystal phase by crystallizing a homogeneous or heterogeneous aluminosilicate gel under hydrothermal conditions at a temperature of about 100.degree. C. (see, or example, D. M. Breck and E. M. Flanigan: "Molecular Sieves", page 47, published by Society of the Chemical Industry in 1968).
Zeolites which have previously been used commercially are in the form of granules, and their function is either an adsorptive separating action or a catalytic action. The adsorptive separation process on granular molecular sieves is a non-continuous batch process involving alternate adsorption and desorption and has low economic feasibility. If there were a membranous article having a zeolite surface, it would be possible to carry out this process continuously on it without attendant changes in phase, and its economic advantage would be greatly increased. So far, however, no such membranous article has been proposed.
On the other hand, if catalysis is effected on the surface of granular zeolite, the reaction product cannot be separated in situ from the reactants, and another separation step is required. In some cases, the reaction product stays on the granular zeolite for a long time, and this causes the defect that the reaction further proceeds to form by-products in addition to the desired main product.
If there were a membranous article having a zeolite surface layer, it would have both a molecular sieve separating action and a catalytic action and would be able to achieve a continuous process without changes in phase. Such a membranous article has not yet been known. Clearly, with granular zeolites, it is virtually impossible to perform adsorptive separation and catalysis at the same time.
Reverse osmosis membranes or ultrafiltration membranes have widely been known as functional membranes. These functional membranes are known as the Loeb-Sourirajan membranes [see, for example, S. Loeb and S. Sourirajan: Advan. Chem Ser., 38, 117 (1963)]. Such anisotropic organic polymeric membranes have come into widespread acceptance, and in recent years, compostie membrance have been known which are obtained by bonding a polymeric material on a previously prepared porous support by such a method as coating or plasma polymerization (see, for example, J. E. Cadotte, et al. "In-situ Formed Condensation Polymers for Reverse Osmosis Membranes", P B Report, November 1972, 229 to 337, NTIS, U. S. Dept. of Commerce). However, the active surface layers of the functional membranes now in use do not have a sufficient separating action on a molecular level, and none of them simultaneously have a catalytic action.
The formation of the active surface layer of the Leob-Sourirajan membranes is based on the theory of coacervation of a solution. Commercially available composite membranes are obtained by forming on a porous support a thin film of a polymeric material having the ability to dissolve selectively a specified component of a feed material.